Handle no blist on non-cpython

blist provides the sortedset class, but it's not currently
compatible with pypy.  This marks blist as a dependency only
when using cpython and gracefully handles blist not being
installed with a warning and the use of 'set' instead of 'sortedset'.

cassandra/cqltypes.py

@@ -20,6 +20,7 @@ import socket
 import time
 from datetime import datetime
 from uuid import UUID
+import warnings
 
 try:
     from cStringIO import StringIO
@@ -35,7 +36,15 @@ apache_cassandra_type_prefix = 'org.apache.cassandra.db.marshal.'
 
 _number_types = frozenset((int, long, float))
 
-from blist import sortedset
+try:
+    from blist import sortedset
+except ImportError:
+    warnings.warn(
+        "The blist library is not available, so a normal set will "
+        "be used in place of blist.sortedset for set collection values. "
+        "You can find the blist library here: https://pypi.python.org/pypi/blist/")
+
+    sortedset = set
 
 try:
     from collections import OrderedDict

setup.py

@@ -1,3 +1,4 @@
+import platform
 import os
 import sys
 import warnings
@@ -146,6 +147,10 @@ def run_setup(extensions):
         kw['cmdclass']['build_ext'] = build_extensions
         kw['ext_modules'] = extensions
 
+    dependencies = ['futures', 'scales', 'blist']
+    if platform.python_implementation() != "CPython":
+        dependencies.remove('blist')
+
     setup(
         name='cassandra-driver',
         version=__version__,
@@ -156,7 +161,7 @@ def run_setup(extensions):
         author_email='tyler@datastax.com',
         packages=['cassandra', 'cassandra.io'],
         include_package_data=True,
-        install_requires=['futures', 'scales', 'blist'],
+        install_requires=dependencies,
         tests_require=['nose', 'mock', 'ccm'],
         classifiers=[
             'Development Status :: 3 - Beta',